1. Field of the Invention
This invention pertains to Schmitt triggers, and more particularly to Schmitt triggers having narrow noise margins and operating at high speed.
2. Description of the Prior Art
The Schmitt trigger is a well known electronic circuit which produces a uniform amplitude output pulse from a random amplitude input signal, and has applications in pulse systems. Schmitt triggers are often used in an input circuit of a large scale integrated circuit ("chip") for receiving a signal from outside the chip and generating in response a signal of uniform amplitude and duration for use by the internal logic of the chip. A typical application of a Schmitt trigger is described in copending and commonly owned U.S. patent application Ser. No. 08/230,045 invented by Sholeh Diba and titled "Integrated Circuit One Shot with Extended Length Output Pulse" incorporated herein by reference.
A prior art Schmitt trigger is shown in FIG. 1. An input signal is provided to input terminal 10 which is connected to the gate electrodes of respectively P-channel transistor Q1, P-channel transistor Q2, N-channel transistor Q3, and N-channel transistor Q4. Each transistor shown in this figure has an associated numeral shown on the figure which depicts the relative size (width) of the gate electrode of that particular transistor. It is understood that the length of each gate electrode is uniform, being in one example 1.0 microns (.mu.m).
As shown, a node between transistors Q2 and Q3 is connected to the output terminal 20, which is also connected to the gate electrode terminal of P-channel transistor Q5 and of N-channel transistor Q6. Also provided is N-channel transistor Q7, the gate terminal of which is connected to a source voltage V.sub.cc. Similarly, transistors Q1, Q2, Q3 and Q4 are connected between the source voltage V.sub.cc and ground (V.sub.ss).
This typical Schmitt trigger produces the output signal waveform shown adjacent to output terminal 20.
The operation of this circuit is as follows. When the output signal at terminal 20 goes high, transistor Q6 turns on (becomes conductive) and hence node A will be held at a high voltage. Thus the output signal at terminal 20 will not be disturbed by any input signal noise. This means that if there is any noise applied to the input signal transistors Q3 and Q4 at terminal 10, this noise would not propagate to the output terminal 20. The same situation applies to transistor Q5 when the output signal goes low. Thus transistor Q5 turns on, while transistor Q7 is on at all times. Therefore node B is pulled down (low) to 0 volts, so that any noise at the input terminal 10 is not able to propagate to the output terminal 20. Thus there must be a significant transition in the input signal in order to pull node B to the voltage of V.sub.cc, or to pull node A to the voltage of V.sub.ss (ground).
The circuit of FIG. 1 has a typical "noise margin" in excess of about one volt. Thus, for example, the low trigger point voltage is 1.7 volts and the high trigger point voltage is 2.7 volts. That is to say, the output signal will be "high" if the input signal voltage (amplitude) exceeds 2.7 volts, and will be "low" if the input signal voltage is below 1.7 volts.
However for a circuit which is intended to operate quickly, i.e. switch quickly in response to randomly varying input signals, it is desirable to have a lower noise margin, for instance, 0.5 volts. Thus, for instance, the low trigger point would desirably be 1.2 volts and the high trigger point 1.7 volts. Thus the noise margin is essentially the amplitude difference between the high and low trigger points. It is also desirable for the Schmitt trigger to respond more quickly to a change in signal amplitude then does the circuit of FIG. 1. These features are especially important when the Schmitt trigger is part of a large scale integrated circuit and associated with an input terminal (pin) of the integrated circuit chip, to improve the speed of the circuit; however, prior art Schmitt triggers do not provide these advantages.